The objective of this research proposal is the study of the functions involved in progression through the G1 and S-phase of the cell cycle in somatic animal cells and the study of the functions regulating gene transfer and gene rearrangements in animal cells. Our work is planned along three main lines of research: 1) The study of gene-transfer in animal cells with particular regard to the state of the transferred gene(s), their modality of integration and expression, and the possibility of carrying them in a plasmid-like state. 2) The identification of the gene affected in some ts growth mutants of BHK cells, and 3) The study of sister chromatid exchange and the possible involvement of this process in gene rearrangements and amplification.